Sensors are used in many automotive applications to sense speeds. One example is an anti-lock brake system (ABS), which uses sensors to sense wheel speed. Though these types of sensors and systems can vary, in one conventional example an encoder wheel is used with a magnetic field sensing system. As the encoder wheel rotates and the teeth or poles of the wheel pass by, the magnetic field sensor can detect the speed of rotation.
It can also be helpful to know the direction of rotation in many applications. For example, modern automobiles often have “hill-holder” systems, which can prevent a vehicle from rolling backwards when the brake is released after being stopped or parked on an incline, or automatic parking systems, which can parallel park a vehicle with minimal driver interaction once activated. In these and other sensor applications, knowing a direction of movement or rotation in addition to the speed of movement or rotation can be helpful or required.
Conventional systems for detecting direction of movement often are complex and/or cannot detect the direction very quickly. For example, one conventional approach uses two magnetic field sensors spaced apart from one another and from an encoder wheel. In operation, two similar but phase-shifted output signals are obtained, one from each sensor, and the direction of rotation can be obtained from the phase relationship between the two signals. Several signal periods are required, however, before sufficient information is available to determine direction of rotation, which is a significant drawback in many applications.